Magnetic recording media generally comprise a magnetizable layer coated on at least one side of a nonmagnetizable substrate. For particulate magnetic recording media, the magnetizable layer comprises a magnetic pigment dispersed in a polymeric binder. The magnetizable layer may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments; and the like.
Some forms of magnetic recording media, such as flexible magnetic recording tape, also have a backside coating applied to the other side of the nonmagnetizable substrate in order to improve the durability, conductivity, and tracking characteristics of the media. The backside coating typically comprises a polymeric binder, but may also include other components such as lubricants; abrasives; thermal stabilizers; antioxidants; dispersants; wetting agents; antistatic agents; fungicides; bacteriocides; surfactants; coating aids; nonmagnetic pigments; and the like.
The magnetizable layer and the backside coating, if any, of a majority of conventional magnetic recording media are derived from materials which require curing in order to provide magnetic recording media with appropriate physical and mechanical properties. To prepare such magnetic recording media, the uncured components of the magnetizable layer or the backside coating, as appropriate, are dissolved in a suitable solvent and milled to provide a homogeneous dispersion. The resulting dispersion is then coated onto the nonmagnetizable substrate, after which the coating is dried, calendered if desired, and then cured.
Curing can be achieved in a variety of ways. According to one approach, the polymeric binder of the magnetizable layer or the backside coating is derived from hydroxy functional polymers which rely upon a chemical reaction between the hydroxy functionality and an isocyanate crosslinking agent to achieve curing. The isocyanate crosslinking agent is typically added to the dispersion just prior to the time that the dispersion is coated onto the substrate.
This approach, however, has a number of drawbacks. For example, the coating will have poor green strength until the cure reaction has progressed sufficiently. As a result, the coating will be susceptible to damage during subsequent processing unless an inconvenient and expensive time delay is incorporated into the manufacturing process. Moreover, after the isocyanate crosslinking agent is added to the dispersion, the viscosity of the solution begins to gradually increase as crosslinking reactions take place. After a certain period of time, the viscosity of the dispersion becomes sufficiently high such that it is then extremely difficult to filter and coat the dispersion onto the nonmagnetizable support.
Radiation curable dispersions have been used as an alternative to isocyanate curable formulations. For radiation curable dispersions, the dispersion is coated onto the substrate, dried, calendered if desired, and then irradiated with ionizing radiation to achieve curing. Radiation curable dispersions are capable of providing, fast, repeatable, controlled crosslinking, thereby eliminating the inconvenient and expensive delays associated with isocyanate curable formulations.
Traditionally, radiation curable formulations have relied upon the reactivity of the carbon-carbon double bonds of acrylates, methacrylates, methacrylamides, acrylamides, and the like to achieve crosslinking. Unfortunately, however, magnetic dispersions prepared from such materials tend to undergo unwanted, premature crosslinking reactions under ambient conditions to form insoluble gels. These dispersions are especially prone to suffer from such premature crosslinking and gellation during dispersion milling. This unfortunate tendency to undergo such premature crosslinking and gellation becomes worse as the weight loading of the magnetic oxide in the dispersion is increased. This problem makes it extremely difficult to manufacture magnetic recording media that are derived from radiation curable materials.
Previous investigators have resorted to the use of free radical scavengers in an attempt to control unwanted crosslinking and gellation of acrylate, methacrylate, and acrylamide materials. However, free radical scavengers provide only limited protection against unwanted crosslinking and gellation inasmuch as the free radical scavenger gets used up in the course of scavenging free radicals.